Micromachines (also called micomechanical devices or microelectro-mechanical devices) are small (micron scale) machines which promise to miniaturize instrumentation in the same way microelectronics have miniaturized electronic circuits. Micromachines include a variety of devices such as motors and gear trains analogous to conventional macroscale machinery. As used herein, the term micromachine refers to any three-dimensional object having one or more sub-millimeter dimensions.
"Stiction" is a fundamental problem associated with micromachines. Stiction most often manifests itself during the structure release process, where liquid capillary forces deflect compliant structures into contact with adjacent surfaces. Subsequent evaporative drying leads to solid bridging due to precipitates left behind from the liquid phase, which usually results in permanent sticking and device failure. This release-related stiction has been successfully circumvented by several methods, including sublimation drying, supercritical drying, and dry-release techniques that avoid the formation of a liquid-vapor interface altogether. In addition, small features have been added to the microstructure perimeter to reduce capillary have been shaping the liquid meniscus. However, these techniques have yet to be widely adopted commercially, either due to process equipment issues or inferior performance in terms of stiction reduction.
The second manifestation of stiction is during actual device operation termed, in-use stiction. When adjacent microstructure surfaces come into contact, capillary, electrostatic, and van der Waals forces may all contribute to adhesion of the surfaces and device failure. None of these techniques mentioned above for the alleviation of release stiction addresses the problem of in-use stiction, which is becoming more important as increasingly compliant microstructures are introduced. Initial attempts to alleviate in-use stiction focused on reducing the contact area between adjacent surfaces by fabricating micro-dimples or by roughening the polysilicon surface on a more microscopic level. While some improvements (a factor of approximately 5) were seen due to roughening, attention has turned more recently towards chemical passivation of the polysilicon surface.
One approach to surface passivation uses self-assembled monolayers (SAMs) constructed on microfabricated devices. This technique is described by Alley, et. al. in U.S. Pat. No. 5,403,665 (the '665 patent). The process of the '665 patent entails an evaporative or supercritical-fluid drying step after the liquid-based processing used to construct the self-assembled monolayer passivation film.
The technology described in the '665 patent solved many problems in the prior art. However, there are still a number of shortcomings associated with the technology of the '665 patent. One problem associated with the '665 patents is that it requires an evaporative drying step. This step constitutes an additional processing step, thus it adds time and expense to the fabrication of a device. In addition, the evaporative drying step proposed in the '665 patent produces little or no improvement over uncoated structures with respect to release related stiction. Other suggested drying approaches such as supercritical-fluid or freeze-sublimation drying require expensive, hard-to-obtain, non-standard processing equipment. In addition, the '665 patent includes processing steps that use chloroform and carbon tetrachloride, substances which are hazardous to health and the environment. As a result, these substances have been banned from industrial use by the Montreal protocol.
In view of the foregoing, it would be highly desirable to provide an improved technique for passivating and drying micromachines. In particular, it would be highly desirable to improve micromachine passivation coatings, improve the drying step associated with the construction of such coatings, and to eliminate the use of environmentally harmful and health-hazardous substances commonly employed in the construction of such coatings.